The developing trend of the DC/DC converter is just like that of the most of the power supply products, towards the direction of high efficiency. Here, the high efficiency is required to be accomplished not only at the rated load but also at a full range of load, that is to say, from the light-load to the full-load. That means higher requirements have been raised on the efficiency of the DC/DC converter, especially on the light-load efficiency of the DC/DC converter.
By employing a structure having plural converters with parallel-connected inputs and parallel-connected outputs and turning off some converters at the light-load, the whole light-load efficiency of the plural converters could be effectively raised and this method is called “phase shedding” controlling method.
FIG. 1 shows a structure having plural DC/DC converters with series-connected inputs and parallel-connected outputs, in which any of the plural DC/DC converters could be a PWM converter or a resonant converter. As shown in FIG. 1, input terminals of converter 1, converter 2 . . . and converter n are series-connected sequentially to receive the input voltage Vin, and the input terminals of the respective converters are parallel-connected to the respective input capacitors C1, C2 . . . and Cn. Output terminals of converter 1, converter 2 . . . and converter n are parallel-connected to the output capacitor Co to provide the output voltage Vo. This structure is suitable for applying to the occasions having high-voltage input and larger-current output. At the mean time, this structure has the feature of automatically balancing the load of each the converter that is each the converter has the same DC component of the input current. But due to the connecting way of series-connected inputs, the phase shedding control could not be applied on the series-connected inputs and parallel-connected outputs converters. This problem is further elaborated as follows using two converters having series-connected inputs and parallel-connected outputs as shown in FIG. 2 for an example.
As shown in FIG. 2, input terminals of converter 1 and converter 2 are series-connected sequentially to receive the input voltage Vin, and the input terminals of converter 1 and converter 2 are respectively parallel-connected to the corresponding input capacitors C1 and C2. Output terminals of converter 1 and converter 2 are parallel-connected to the output capacitor Co to provide the output voltage Vo. Iin1 and Iin2 are the DC components of the input currents of converter 1 and converter 2 respectively. Vin1 and Vin2 are voltages across input capacitors C1 and C2 respectively and the DC components of Vin1 and Vin2 are expressed as Vin1_d and Vin2_d respectively. Io1 and Io2 are output currents of converter 1 and converter 2 respectively. Under this kind of circuit structure, each converter has its normal operating range according to the design requirements. And each converter has its input voltage range in which it can operate normally. The input voltage range comprises a maximum value Vmax and a minimum value Vmin. The voltage input range is obtained according to the input and output design requirements of each converter and the stress requirements of the elements of each converter.
Under the circumstances that both of the two converters operate normally and stably, Iin1 is equal to Iin2; the DC components of currents flowing through capacitors C1 and C2 are both zero; and the DC voltages Vin1_d and Vin2_d are kept unchanged. If under a certain load condition, e.g., under the light-load condition, converter 1 is turned off, then Iin1 is equal to 0 and direct currents flow through capacitors C1 and C2 to charge capacitor C1 and discharge capacitor C2. Thus, Vin1 is raised and Vin2 is reduced. When Vin2 is lower than the minimum voltage value Vmin of the converter 2, neither converter 1 nor converter 2 could operate normally.
Then a novel controlling method for the structure of multi-converter having series-connected inputs and parallel-connected outputs is provided to avoid the happenings of the aforementioned problems and to effectively raise the efficiency under a certain load condition, e.g., the light-load condition.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicant finally conceived a controlling method for a multi-converter structure having series-connected inputs and parallel-connected outputs.